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Description

Joshua Mayersky1 Rashmi Jha1

1, University of Cincinnati, Cincinnati, Ohio, United States

As a transition metal dichalcogenide, tungsten disulphide exhibits desirable characteristics in the mono to few layer form factor as opposed to its bulk form. One of these properties is a shift from an indirect bandgap of 1.2eV in the bulk to a direct bandgap of 1.8eV in the monolayer form. This enables mono to few layered tungsten disulphide to be semiconducting as well as photo conducting. Traditional methods of fabrication for monolayered tungsten disulphide such as chemical vapor deposition involve high temperatures, rendering them incompatible with processing on flexible substrates, and they have a relatively low yield. While mechanical exfoliation produces high quality monolayered single crystals at reasonable temperatures, it also suffers from the low yield problem. Liquid exfoliation is a viable alternative here, as it can produce both high volumes of mono to few layered flakes, and is compatible with deposition and processing on flexible substrates via printing techniques like ink-jet printing. In this work, first, a printable tungsten disulphide based ink is developed from readily available tungsten disulphide powder (0.6μm average particle size), and then an ink-jet printing based deposition method for a tungsten disulphide film is presented. In terms of developing a printable ink, optimization of dispersed monolayered flake concentration is discussed as well as characterization of said flakes. Thereafter, printing parameters and optimization of printed line characteristics is studied. These line characteristics include constraining the volume of ink deposited to the desired dimensions, tungsten disulphide flake coverage, and bulk electrical characteristics.

Characterization of inks is performed by optical UV-Vis spectrometry using a Perkin-Elmer spectrometer, and the presence of exciton absorbance peaks are confirmed and analyzed. Metrics using the A-exciton peak generated by the few-layered flakes are used to calculate the average flake lateral dimensions, the concentration of tungsten disulphide in the inks after size selection and filtering, as well as the average monolayer count of the flakes. After printing, scanning electron microscopy with a FEI XL-30 is used to confirm average flake lateral size and average flake area coverage, while a Veeco Dimension 3100 atomic force microscope is used to confirm flake thickness. Tungsten contacts are deposited by a Denton Discovery 24 sputtering system and patterned via shadow mask. Electrical characterization is performed using a Keithley 4200 semiconductor characterization system to understand conductivity and charge transport properties. We believe that this work will lay a solid foundation for developing printable tungsten disulphide inks from low-cost powders and understanding the electrical, optical, and physical characteristics of printed tungsten disulphide films.